This invention relates to particle size measuring instruments in general and in particular to devices for checking the calibration, sensitive volume, and particle counting efficiency of optical, single particle size spectrometers.
During the last decade or so, instruments have become commercially available for measuring the size of individual, microscopic, airborne particles such as cloud droplets or other particulate aerosols. These instruments, called particle size spectrometers, are generally based on the principle of visible light scattering by particles aspirated through an illuminating beam. For particles larger than about 20 .mu.m diameter some spectrometers are based on light interception or shadow imaging techniques.
In the light-scattering models, light scattered out of the illuminating beam by passing aerosol particles is collected by a mirror or lens and directed to a sensitive photodetector. The photodetector current pulses are amplified and, by means of electronic pulse height analysis techniques, they are sorted and counted in histogram form. The number of counts indicates the particle concentration and the histogram provides a size frequency distribution for the detected particles. A given spectrometer will respond to particles within some preselected interval of sizes, such as 1 to 30 .mu.m diameter, for example. The histogram readout further divides this interval typically into five to fifteen sub intervals or size categories.
A problem common to all such spectrometers is the need for calibration so that the instrument will properly indicate the size of each validly sampled particle. A good primary calibration requires so much care and special equipment that practically all instrument users rely on the manufacturers to provide such services. For spectrometers sensitive to particles larger than about 10 .mu.m diameter, a rough calibration can be performed in the field with the use of commercially available, microscopic glass beads. However, the available beads are sufficiently varied in size within a sample that the finer size resolution capabilities of some instruments cannot be properly tested.
Related, important problems include the need for measuring the optical depth of field (the length of that portion of the illuminating beam which is sensitive to transiting particles) and the efficiency of the instrument in registering every validly sampled particle.
The prior art for accomplishing these objectives is described below, along with the disadvantages inherent in the prior art.
a. Primary Calibrations
A primary calibration basically consists of adjusting amplifier gains and or noting the size categories into which appear particle counts from a sample of known, uniform (monodisperse) sized particles of the type to be measured in application. By selectively changing the monodisperse size of the test particles, the total instrument response (a determination of the particle diameters corresponding to each of the available size categories for the instrument) can be obtained.
The principal difficulties involved here are in the production of test particles of a known and stable, monodisperse size. For most common applications water droplets would be the preferred test particles but monodisperse droplets are difficult to produce in the diameter range of 0.1 to 10 .mu.m covered by a majority of the optical, single particle spectrometers in use. Even if a reasonable monodispersity can be initially achieved by some type of droplet generator, it is difficult to prevent droplets of this small size from evaporating partially during the transit from the generator to the sensitive volume of the particle sizing instrument.
To avoid these difficulties, various types of microscopic latex spheres, oil droplets, or other non volatile particles are generally used. However, the special equipment (nebulizers, air pumps, drying chambers, etc.) required is sufficiently elaborate and the procedures sufficiently tedious that most instrument owners rely upon the manufacturers to provide this primary calibration service. Such equipment is definitely impractical for routine use in the field.
The only existing method that is simple enough for use in the field involves the use of microscopic glass spheres or "beads." These are commercially available but are not very practical in sizes smaller than the nominal 13 .mu.m diameter size class produced by the principal manufacturer. The available sizes are not very monodisperse either, with the result that the range of bead diameters is wider than the resolution capabilities of some instruments. Other difficulties with the glass bead method are:
First, the test results are generally not as reproducible as is desired for an accurate calibration check.
Second, for instruments not equipped with an internal blower or pump for pulling sample air through the sensor unit, extra blowers, pumps, connecting tubes and fittings are necessary for preparing the probe for the calibration checks.
Third, since the microscopic beads have the consistency of a fine powder which almost floats in the air, the procedure must usually take place inside a room or other shelter that is free from drafts or air currents that interfere with the dispensing and use of the beads, and thus,
Fourth, the procedure usually requires dismounting the probe from its normal outdoor sampling location (ship, tower, aircraft, roof, etc.), carrying it inside along with its power and data cables or some auxiliary set of cables, and generally proceeding through an annoying, if not difficult, exercise.
For the beam attenuation or shadow imaging type of particle size spectrometers a calibration technique involving the use of opaque wires has recently been disclosed in U.S. Pat. No. 4,135,821 entitled "CALIBRATION OF OPTICAL PARTICLE SIZE ANALYZER" issued in the names of applicants William H. Pechin, Louis H. Thacker, and Lloyd J. Turner. It must be pointed out here that the translucent fibers of the present invention are suitable for use with these types of analyzers as well as with the light-scattering types, but the opaque wires are not. That is, translucent fibers of an adequate diameter will cast a measuring shadow as will opaque wires, but the latter will not scatter light in a way that is uniquely related to the wire diameter. Light-scattering spectrometers are designed to measure the intensity of light deflected out of the beam, usually in a near forward direction, by passing particles. For the microscopic particles of interest, this deflection is due to both scattering and refraction by the translucent particles. The resultant deflected intensity is a unique function of the particle diameter, index of refraction, wavelength of the light in the illuminating beam, and the angle of deflection. Opaque objects can only reflect and/or absorb the illuminating beam, and the reflection mechanism does not deflect light in a way that is unambiguously related to the diameter of the object.
For these reasons it is to be further emphasized that the invention of an opaque wire technique for performing primary calibrations on beam attenuating type analyzers does not imply the use of translucent fibers as an obvious technique for calibrating light-scattering spectrometers. The latter are based on the teachings of light-scattering theory as applied to spherical particles. The theory is very complex, mathematically, even when applied to this simplest of symmetries--the sphere. Non-spherical particles not only enormously complicate the mathematics of the theory but are also unsuitable for use with existing particle size spectrometers. This is because the intensity of light scattered into the detector by non-spherical particles depends on the shape of the particle and on the angle the particle symmetry axes make with the illuminating beam as the particle passes through. Since this angle is generally random there is no longer a unique relationship between the detected light signal and the size of the particle as there is for spherical particles. Fortunately, many of the naturally occurring particles of interest are spherical in shape. Thus, those skilled in the art of light-scattering particle size spectrometry have logically chosen spherical test particles for use in the calibration and testing of these instruments. Calibration tests have therefore been thought of only in terms of primary calibrations since the test particles were of the same size and shape as the particles to be measured in applications. Until the disclosure of this present invention, it was apparently not realized that aligned, translucent, cylindrical fibers could be used in place of microscopic spheres, not as primary calibration particles but as secondary, or transfer standards for checking or adjusting the calibration once a primary calibration with translucent, microscopic spheres had been performed.
b. Determination of the Optical Depth of Field
In addition to accurate particle size determination, practical use of the data obtained with these instruments requires a knowledge of the particle number density--the number of particles per unit volume of the sampled air. This information is easily available if the air samples are aspirated through the probe at a known flow rate and if the sensitive volume of the illuminating beam is known. The sensitive volume is simply the product of the effective beam width and the depth of field (d.o.f.) determined by the imaging optics and the detection system in the instrument. The d.o.f. is preset by the manufacturer but since it may be defined and controlled by optical and electronic components it is subject to change if these components change or drift.
The prior art for measuring or checking the position and length of the d.o.f. involves the insertion of a suitable, usually static, test object in the particle illuminating beam while an electrical or optical response somewhere in the instrument is monitored. The disadvantages of the prior art are:
First, the particle analyzer must usually be opened up to measure voltages on electronic circuits that are normally inaccessible, and therefore,
Second, the probe must usually be dismounted from its normal, outdoor sampling location as in the case of prime calibration checks.
Third, the measurements usually require an oscilloscope or other laboratory instruments which therefore make field checks less practical.
c. Efficiency Checks of Particle Counting Circuitry
It is not unusual that during extended field deployments of optical particle counters the indicated particle counts may sometimes decrease significantly without any apparent reason. Sometimes this decrease is a natural result of unusual atmospheric conditions, for example, but it can also be due to instrument malfunction.
There is no known prior art that is capable of providing an exactly countable sequence of individual test particles with diameters less than about 100 .mu.m for testing the counting efficiency of optical scattering spectrometers.